The eutectic structure of aluminum silicon alloys has long been studied to determine the mechanical properties of the alloys, see U.S. Pat. Nos. 1,387,900 and 1,410,461. After more than 80 years of studying this eutectic structure, those skilled in the art now understand that sodium or strontium additions to the eutectic melt in only 100 ppm concentrations changes the size and morphology of the eutectic silicon phase resulting in a significant increase in the alloy's ductility.
Still, hypereutectic aluminum silicon alloys are not used to a great extent in sand casting processes because they are difficult to machine and because the primary silicon particle size is larger at sand casting cooling rates than at cooling rates for casting processes that use metal molds. As a result, there is a requirement to control the casting's microstructure in order to achieve an acceptable machinability. Achieving an acceptable machinability in a hypereutectic alloy is typically accomplished through phosphorus additions to the alloy melt to refine the primary silicon particle size. However, phosphorus prefers to form phosphides with common melt additives such as strontium and sodium rather than reacting with aluminum to form aluminum phosphide. This is problematic because aluminum phosphide is the nucleus for primary silicon formation in the eutectic structure of hypereutectic aluminum silicon alloys. Accordingly, the eutectic structure of phosphorus containing hypereutectic aluminum silicon alloys is always unmodified.
Thus, phosphorus refined, solution heat treated, quenched and aged, hypereutectic aluminum silicon structures provide the baseline for machinability, yet this baseline generally requires diamond tooling for proper machining. In contrast, eutectic aluminum silicon alloys and hypoeutectic aluminum silicon alloys, where the eutectic silicon structure is modified with strontium or sodium additions, have increased ductilities and are easier to machine. However, when the modified eutectic in the hypoeutectic alloy structures are compared to unmodified structures, the strontium or sodium modified eutectic structures exhibit nearly identical machinability in the heat treated condition with the unmodified structures. It is believed that this equivalence in machinability is due to the eutectic silicon phase occurring as a continuous phase in the eutectic whether the eutectic is modified or unmodified. Further, since it is always easier to machine the less ductile T6 or T7 heat treated condition, compared to the as cast condition, there is an effect that base metal properties have on machinability that is quite significant. Accordingly, there is not a predictable treatment that improves machinability of hypereutectic aluminum silicon alloys.
Hypereutectic aluminum alloy B391 (AA B391) includes 18 to 20% silicon by weight for wear resistance, 0.4 to 0.7% by weight magnesium for aging response to increase strength and has maximums for iron and copper of 0.2% by weight for good sand casting attributes, and is the only hypereutectic aluminum silicon alloy registered for sand casting by the Aluminum Association. The 0.2% by weight maximum copper constituency ensures that (for any given silicon content), the solidification range, that is, the temperature difference between the liquidus and solidus, is at a minimum. In comparison, AA 390 has the same range of elements as AA B391, except AA 390 has 4.5% by weight copper constituency. Thus, the narrow solidification range of AA B391 occurs primarily because the significantly lower copper constituency raises the solidus melting point by nearly 100° Fahrenheit compared to AA 390.
The narrow solidification range of AA B391 is important because the primary silicon, which is less dense than the molten alloy, it is less likely to float and segregate upon precipitation in an alloy of narrow solidification range. The low iron and manganese contents of AA B391 are desirable and are particularly attractive for a sand cast hypereutectic aluminum silicon alloy that solidifies slowly. The mechanical properties of AA B391 are significantly degraded when the iron phase grows large during the slow cooling, because a needle like morphology results for the iron phase, degrading mechanical properties.
Historically, nickel was an essential element in Y alloy (4% by weight copper, 2% by weight nickel, 1.5% by weight magnesium, balance aluminum), developed during World War I. Nickel is present in only three registered alloys with the Aluminum Association today in concentrations between 2% and 3% nickel. Thus, it is known to use nickel as a minor constituent in some aluminum copper alloys, such as AA 242, AA 336 and AA 393, wherein the element imparts high strength at high temperature. AA 242 has a formulation of 3.7 to 4.5% by weight copper, 1.2 to 1.7% by weight magnesium, 1.8 to 2.3% by weight nickel and balance aluminum. AA 336 has 11 to 13% by weight silicon, 1.2% by weight maximum iron, 0.5 to 1.5% by weight copper, 0.7 to 1.3% by weight magnesium, 2.0 to 3.0% by weight nickel and balance aluminum. Similarly, AA 393 has a hypereutectic formulation of 21 to 23% by weight silicon, 1.3% by weight maximum iron, 0.7 to 1.1% by weight copper, 0.7 to 1.3% by weight magnesium, 2.0 to 2.5% by weight nickel and balance aluminum.
Additionally, more than forty years ago, there was considerable interest in the Al—NiAl3 eutectic, unidirectionally solidified, as a fiber reinforced material, especially for high temperature applications. As identified in the reference to B. K. Agrawal, Met A 6, 152605, in the book, Aluminum Alloys: Structure and Properties by L. F. Mondolfo page 339 (Butterworth Publications Ltd, 1976), by directional freezing, the eutectic may be made to crystallize with the NiAl3 fibers aligned in the direction of growth, with the spacing between the fibers dependent on the freezing rate. The same reference indicates that additions of barium, cerium and cesium to the unidirectionally solidified Al—NiAl3 eutectic changes the solidification pattern from colony to dendritic It is also known that aging after quenching from high temperature does not produce hardening of binary Al—Ni alloys to be of practical use.
However, the addition of nickel in concentrations approaching 6% to aluminum silicon magnesium casting alloys, aluminum silicon copper casting alloys, aluminum silicon copper magnesium alloys or aluminum copper casting alloys have not been studied. This is because it is known that nickel additions of 2% by weight or less have the effect of reducing hot shortness in some castings and also have the effect of reducing the coefficient of thermal expansion.
Additionally, U.S. Pat. No. 6,168,675 describes a hypereutectic aluminum silicon alloy having 2.5 to 4.5% by weight nickel, but with a very high manganese content of 1.2% maximum by weight and a very high iron content of 1.2% by weight maximum. This alloy is intended for the die casting process or permanent mold casting process to make vehicular disk brake components. Because of the high manganese and iron contents, this alloy has a very high heavier metal content that requires a high holding temperature to prevent the heavier metals from dropping out. Furthermore, the high manganese content is necessary to modify the needle like beta iron aluminum phase to the alpha iron aluminum phase and increases the yield strength, tensile strength and elongation, both at ambient and high temperatures. Notwithstanding the attributes imparted to the alloy from high levels of manganese and iron, the alloy of U.S. Pat. No. 6,168,675 would not be suited for a slow cooling process like sand, lost foam or investment casting because the large needle like iron phase particles would form, even with the high levels of manganese, thereby hindering feeding during solidification which results in increased porosity levels and decreased ductility levels.
Sand casting processes are increasingly being used to cast complex metal products. Sand casting procedures include lost foam casting, lost foam with pressure casting, green sand casting, bonded sand casting, precision sand casting and investment casting. Perhaps the most beneficial and economical of these types of castings is lost foam casting with pressure. Such a method is described in U.S. Pat. No. 6,763,876 entitled Method And Apparatus For Lost Foam Casting Of Metal Articles Using External Pressure, the subject matter of which is incorporated herein by reference. All of the above discussion does not mean that die casting alloys containing nickel or nickel-free cannot be made more machinable if the primary silicon particle size is small and the eutectic is modified, which has not heretofore been demonstrated.